This invention relates to central station systems such as central station alarm systems, and more particularly to improved transmission apparatus for such systems.
Central station systems typically include a central station and a plurality of remote stations connected to the central station by a transmission line or loop. The system can be constructed so that a remote station spontaneously transmits coded signal information to the central station representing conditions at the remote station, or the central station may be periodically interrogate the remote stations by transmitting coded signals identifying the remote stations in turn and each remote station responds by transmitting coded signals indicating conditions at that remote station. The central station may utilize the information received from the remote stations in a variety of ways. For example, the central station may decode and display that information for the operator of the system, or the central station may initiate or perform other operations appropriate to the condition of the remote stations. The central station may transmit additional signals to a remote station causing apparatus at the remote station to initiate or perform certain operations to alter conditions at the remote station.
Central station systems are used in a variety of applications. Central station alarm systems are used, for example, to monitor one or more alarm conditions (e.g., smoke, fire, burglary, power loss, refrigeration loss, etc.) at a plurality of remote locations and to transmit signals indicating the occurrence of such an alarm condition either spontaneously (i.e., when the alarm condition occurs) or when interrogated as mentioned above. This information is received by the central station which typically decodes and displays the information for the operator of the system who then initiates action appropriate to the alarm condition detected (e.g., advises police or fire authorities of the occurrence and location of the alarm condition). Central station systems may also be used to monitor conditions throughout an electric power distribution system, a pipeline system, or any other similar system. Central station systems may be used for process monitoring and control (e.g., chemical process monitoring and control) where conditions at a number of physically remote locations must be monitored and possibly controlled.
The size of a central station system may vary considerably. A central station alarm system, for example, may include remote stations at a plurality of geographically remote locations or premises. On the other hand, such a system may service only a single plant, or building. For example, remote stations may be located on the several floors of a high-rise office building and the central station may be located on the main floor of that building.
Although the present invention will be described herein in its application to central station alarm systems for use in a single plant or building, the invention is not limited to such systems, and it will be understood that the principles of the invention are applicable to central station systems generally, including any of the various types mentioned specifically above.
Any of a variety of faults can occur in the transmission line connecting the central station with the remote stations in a central station system. For example, a break in one of the transmission line wires or the grounding of one of those wires can disable the entire system unless some form of protective device is provided. U.S. Pat. No. 253,080 issued to C. F. McCulloh on Jan. 31, 1882, shows a system which can continue to operate despite a break in or grounding of a transmission line wire. There are, however, other types of fault conditions (e.g., wire to wire short circuits) which can occur, particularly in more sophisticated systems requiring transmission lines having two or more wires. Protection against these other types of fault conditions, as well as against broken or grounded wires, is therefore required.
It is accordingly an object of this invention to provide improved transmission apparatus for central station systems.
It is a more particular object of this invention to provide transmission apparatus for central station systems which can continue to operate despite the occurrence of any of several types of fault conditions in the transmission line, including a broken wire, a grounded wire, a wire to wire short circuit, and various combinations thereof.
It is a further and more particular object of this invention to provide transmission apparatus for central station systems which includes means for indicating which of the several types of fault conditions mentioned above is occurring when such a fault condition occurs.
It is still another more particular object of this invention to provide transmission apparatus for central station systems which includes means for modifying the transmission circuits to maintain communication between the central and remote stations when a fault condition occurs which would otherwise interrupt said communication.
Certain aspects of this invention are applicable to systems in which information is transmitted alternately from the central station to the remote stations and from the remote stations to the central station (two-way transmission). Other apsects of the invention are applicable both to those systems and to systems in which information is transmitted only from the remote stations to the central station (one-way transmission). The invention will be described in its application to two-way transmission systems, it being understood that if one-way transmission is all that is required, the portion of the apparatus provided for transmission of information from the central station to the remote stations can be omitted.